1. Field of the Invention
The invention proceeds from a current transformer for a metal-enclosed gas-insulated high-voltage installation accordance with the preamble of claim 1 or of claim 9.
2. Discussion of Background
In this connection, the invention relates to a prior art as disclosed, for example, in the publication EP-A2-0 522 303.The combined instrument transformer described therein is intended for a metal-enclosed gas-insulated high-voltage installation. It has a current sensor and a voltage sensor and a signal processing system downstream of the current sensor and voltage sensor. The current sensor used is a pickup coil which is wound around an annular core composed of nonferromagnetic, predominantly isotropic material. Provided as voltage sensor is a hollow-cylindrically formed measuring electrode. After the current sensor and the voltage sensor have been installed in the metal enclosure, the coil and the measuring electrode concentrically surround a current conductor of the gas-insulated switching installation.
The pickup coil is embedded in an annular insulating body which is disposed in an annular space. The walls of the space are composed of electrically conducting material, preferably of an aluminum alloy. The insulating body is fixed in position in said space. On its circumferential surface adjacent to the current conductor, the space is bounded by an areal, annular screening electrode. One end face of this screening electrode is connected in an electrically conducting manner to the wall on one side of the annular space, while the end face situated opposite it is connected in an electrically insulating manner to the wall on the other side of the annular space. Provided here as insulating connection is a comparatively narrow gap completely filled with insulating material. The walls of the annular space form an electrically conducting screen with the screening electrode. Said screen protects the annular space against the influence of electrical interference fields. During measurement in the high-voltage installation under operating conditions, the pickup coil therefore delivers signals which are free of influences due to the electrical interference fields. The pickup coil provides signals which are proportional to the magnitude and the change with respect to time of the current flowing in the current conductor of the gas-insulated switching installation and which are virtually free of the effects of electrical extraneous fields because of the suitably disposed screen. For many practical applications, a current transformer constructed in this manner is probably completely satisfactory.
However, the gap filled with insulating material in the screen surrounding the pickup coil is the reason for rapid and very rapid transient electrical interferences, for example in a frequency range from more than 1 MHz to approximately several GHz, not being isolated from the pickup coil. Such interferences distort the measured signal and they may additionally be the reason for the overvoltage protection always present at the input of the signal processing system being overloaded.